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Groups of Efficient Nuclei in Dust-Free Air. 297 



Art. XXXII. — On Groups of Efficient Nuclei in Dust-Free 

 Air : by C. Barus. 



1. Dust free Air. — By this term I refer to atmospheric air 

 filtered with extreme slowness (through large wide filter of 

 packed cotton) and thereafter left without interference for 

 two or more hours. Such air shows a high fog limit. In the 

 fog chamber used the coronal condensation begins at a pressure 

 difference of about 8j> = 26 cm., rain-like condensation at 

 8p = 21 cm. 



In the present experiments all tests are made at Sp = 41*5 cm., 

 at a pressure difference therefore much above the fog limit, 

 and probably approaching the condensing power of the appa- 

 ratus. The number of nuclei computed from the coronas 

 observed is an approximation merely, as the constants needed 

 for the very large range of variation in question are not avail- 

 able. Nevertheless, if the same Sp is used throughout, the 

 nucleations obtained are immediately comparable. With these 

 reservations" the number of nuclei found in the dust-free air 

 and at the Sp in question is about 3S0 X 10 3 to 100 X 10 3 per 



cm 3 . It is obvious, more- 



over, that these nuclei 

 are excessively small, 

 much smaller than ions, 

 smaller even than those 

 which would respond to 

 smaller exhaustions, 

 exceeding Sp = 26 cm. 



In figure 1 I have 

 given an example of these 

 relations. Between 

 Sp = 21 and 26 (for this 

 apparatus) condensation probably takes place largely on ions, 

 above that on the nuclei of dust-free air. The upper clotted 

 line shows the limit of value found, the latter being variable 

 because (as will appear more clearly below) the ionization of 

 atmospheric air is essentially variable. Though relatively 

 small in number, the ions from their larger size probably cap- 

 ture much of the moisture. 



2. Effect of Radium. — Xow let the fog-chamber (fig. 3) be 

 subjected to the radiations from weak radium (10,000 X, 

 10 mg.) contained in a thin hermetically sealed aluminum 

 tube. As the walls of the fog-chamber are *3 cm. thick and 



* The nuclei are supposed to be removed by exhaustion, faster than they 

 can be restored, either by radiation or by the molecular mechanism. 



